1. Technical Field
The disclosure relates in general to a treatment system and a method, and more particularly to a groundwater treatment system and a method for degrading chlorinated dense non-aqueous phase liquids (DNAPLs) effectively.
2. Description of the Related Art
Halogenated hydrocarbons are commonly found in the environment because they are widely used as an effective, yet relatively nonflammable, solvent, unlike kerosene or gasoline. Halogenated hydrocarbon solvents such as trichloroethene (or trichloroethylene, TCE) and tetrachloroethene (or perchloroethylene, PCE), which have higher densities than water, are referred to as dense non-aqueous phase liquids (DNAPLs). Groundwater contamination by DNAPLs, like TCE and PCE, were identified as a serious problem beginning in the 1970s. However, some groundwater contaminated sites are either technically impracticable, or the concentrations of the chlorinated organic contaminants are still higher than the regulated maximum contaminant levels even after decades of remediation.
Small amounts of DNAPLs can serve as a long term contamination source because their water solubility is low and they are toxic at very low concentrations. Superfund sites with TCE and PCE groundwater contamination include: electronic manufacturing plants, military facilities, drycleaners and old landfills. In most cases groundwater pollution resulted from storage or disposal of liquid solvent wastes between 1940 and 1970. Often TCE and PCE are found in water with other toxic chemicals such as dichloroethenes (DCE) and vinyl chloride (VC) that are produced by natural degradation of these two chemicals. TCE is considered an animal carcinogen and a health hazard to humans. The International Agency for Research on Cancer has determined that trichloroethylene is “probably carcinogenic to humans”. People exposed to TCE by contaminated drinking water exhibit health problems including skin irritations, cancers, birth defects, miscarriages, and coordination, speech and hearing impairment.
Many development and implementation of groundwater contamination remediation treatments have been provided. The treatment methods for DNAPL contaminated groundwater generally can be classified into passive methods and active methods. Passive methods, such as permeable reactive barriers (PRB) or hydraulic isolation, assure clean water on the down gradient side. However, as long as the DNAPL is not removed or depleted in the groundwater, the treatment operation has to keep running, which often takes decades or even longer. Since the long-run maintenance and management poses uncertainties to the quality of the treatment, actively removal of the DNAPLs often is desired.
The common active treatment methods usually involve injection of treatment chemicals, such as persulfate and permanganate. These chemicals are usually very effective in degrading the dissolved contaminants. However, the depletion of DNAPLs is limited by the slow dissolution process. Therefore, these chemicals are not effective in depleting the DNAPLs.
Nowadays, more focuses are placed on developing treatment chemicals targeted at DNAPLs. The emulsified zero-valent irons, known as EZVI, formulated by the National Aeronautics and Space Administration (NASA) is one example. The zero-valent irons have been well known as an environment-friendly treatment agent, capable of reductive dechlorination and thus detoxication for the chlorinated hydrocarbons, little chlorinated byproducts, and fast reaction rate and inexpensive. Based on a well designed recipe, zero-valent irons, vegetable oil and water are emulsified by the surfactants to form the EZVI, whose continuous phase is water. According to NASA, the oil membrane increases the chance of zero-valent irons remained on DNAPLs, and also serves as the carbon sources for microorganisms; thereby treating DNAPLs by bioremediation after chemical treatment. EZVI has been applied in a pilot-scale remediation. It was observed that the emulsion system was not sustained shortly after injection into the groundwater, leaving the irons trapped in soil and the oil floating at the water table.
The other targeted treatment chemical is proposed by Prof. Lowry's research group of Carnegie Mellon University, who use copolymers as the dispersants for nanoscale zero-valent irons in water. The copolymers consist of two hydrophilic polymers and a hydrophobic one. It was found that the hydrophobic polymer helps the zero-valent irons being retained at the DNAPLs/water interface. Despite of the improvement, the degradation reaction is still limited to the DNAPLs/water interface.